The present invention relates to an image projection apparatus referred to as a liquid crystal projector or the like.
A liquid crystal display element for use in liquid crystal projectors is formed, for example, by sealing nematic liquid crystal having a positive dielectric anisotropy between a first transparent substrate having a transparent electrode provided thereon and a second transparent electrode having a transparent electrode forming a pixel, wiring, a switching element and the like provided thereon. Such a liquid crystal display element includes liquid crystal molecules such that the arrangement of their long axes is continuously twisted 90 degrees between the two glass substrates and thus is called a TN (Twisted Nematic) liquid crystal display element.
In the TN liquid crystal display element, a voltage is applied between the first transparent substrate and the second transparent substrate to change the director orientations of the liquid crystal molecules to vary retardation (phase difference) in the whole liquid crystal layer, thereby achieving optical modulation.
More specifically, the arrangement of the liquid crystal molecules is continuously twisted 90 degrees between the two glass substrates while the lowest voltage is applied to the liquid crystal layer, for example, while no voltage is applied (hereinafter referred to as a “no-voltage application state” for convenience). In this state, when polarized light having a polarization axis in a predetermined direction enters the liquid crystal display element, the light is provided with retardation and the polarization direction is rotated approximately 180 degrees. As the voltage applied to the liquid crystal is increased, the directors of the twisted liquid crystal molecules are oriented in the thickness direction of the liquid crystal layer to reduce the retardation provided for the polarized light having the polarization axis in the predetermined direction. Such a liquid crystal display element is called a normally white (mode) liquid crystal display element.
Besides the abovementioned TN liquid crystal display element, a so-called VAN (Vertical Arrangement Nematic) liquid crystal display element is currently used. The VAN liquid crystal display element is formed by sealing nematic liquid crystal having a positive dielectric anisotropy between a first substrate having a transparent electrode provided thereon and a circuit substrate having a second substrate arranged in the form of pixels as a two-dimensional optical switch, wiring, a switching element and the like provided thereon. The nematic liquid crystal molecules are arranged in homeotropic alignment such that the long axes thereof are approximately perpendicular to the two substrates.
In such a VAN liquid crystal display element, a voltage is applied to the first substrate and the second substrate in the form of pixels, that is, between the opposite electrodes, to change the director orientations of the liquid crystal molecules to vary retardation in the whole liquid crystal layer, thereby realizing optical modulation.
More specifically, in the no-voltage application state of the liquid crystal layer, the long axes of the liquid crystal molecules are approximately perpendicular to the two substrates in the homeotropic alignment. When polarized light having a polarization axis in a predetermined direction enters the liquid crystal element, the light is provided with little retardation. As the voltage applied to the liquid crystal is increased, the arrangement of the director orientations of the liquid crystal molecules is continuously twisted between the two substrates to increase the retardation provided for the polarized light having the polarization axis in the predetermined direction. Such a liquid crystal display element is called a normally black (mode) liquid crystal display element.
In recent years, with a higher degree of resolution and a reduced size of the liquid crystal display element, pixel electrodes each forming pixel areas are arranged with extremely small intervals therebetween, and it has been found that a lateral electric field produced between adjacent pixel electrodes adversely affects the alignment of the liquid crystal.
Especially, advanced liquid crystal projectors have pixels with a very small size of approximately 10 μm, so that the influence of the lateral electric field is not ignorable. Now, a specific example of the disadvantage due to the lateral electric field will be described with reference to FIG. 13 which shows an exemplary VAN reflective liquid crystal display element.
In FIG. 13, reference numeral 1 shows a glass substrate having a transparent electrode provided thereon, and reference numeral 2 shows reflective pixel electrodes. Of the reflective pixel electrodes 2, reference numeral 2a shows a reflective pixel electrode in a black display state to which no voltage is applied, and reference numeral 2b shows a reflective pixel electrode in a white display state to which a voltage is applied. Reference numeral 3 shows a liquid crystal layer, and 4 a liquid crystal molecule in which the major axis of the ellipse corresponds to the director orientation in the liquid crystal molecule. Reference numeral 5 shows the transparent electrode, 6 an alignment film evaporated on the transparent electrode 5, and 7 an alignment film evaporated on the reflective pixel electrodes 2. Reference numeral 8 shows distribution of reflectance when the liquid crystal display element is sandwiched between crossed-Nicol polarizing plates, not shown. Reference numeral 9 shows a lateral electric field produced in the liquid crystal layer 3.
The VAN reflective liquid crystal display element uses the ECB (Electrically Controlled Birefringence) effect to totally control the polarization state of light traveling through the liquid crystal layer 3.
As shown in FIG. 13, the lateral electric field 9 is produced near the boundary between the pixel 2a to which no voltage is applied and the pixel 2b to which a voltage is applied, which causes an area 10 in the pixel of the reflective pixel electrode 2b where the alignment of the liquid crystal molecules is bad. This makes it impossible to control the polarization with the ECB effect to reduce the reflectance in the area 10. In other words, the reflectance in the whole pixel of the reflective pixel electrode 2b is reduced.
Japanese Patent Laid-Open No. 10(1998)-161127 has disclosed a liquid crystal display element capable of reducing bad alignment of liquid crystal molecules due to such a lateral electric field (disclination).
In the reflective liquid crystal projector, a color separation/combination optical system is used to separate white light from a light source into light components in three wavelength bands of a red band, a green band, and a blue band, and the light components in the respective wavelength bands are directed to three liquid crystal display elements associated therewith. After modulation by the three liquid crystal display elements, the light components are combined by the color separation/combination optical system before projection. Now, description will be made of characteristics when the same voltage is applied to all of the reflective pixel electrodes 2 of the reflective liquid crystal display element in such a reflective liquid crystal projector (hereinafter referred to as “in an all pixel display state”). In other words, description will be made of the case where the same voltage is applied to the reflective pixel electrodes 2a and 2b in FIG. 13.
Typically, liquid crystal molecules have a wavelength dispersion characteristic of refractive index anisotropy Δn, and to provide the highest reflectance generally in crossed Nicols, a higher voltage needs to be applied to the liquid crystal as the wavelength is longer. FIG. 14 shows the dependence of the reflectance on the voltage for each wavelength band in a typical VAN reflective liquid crystal display element in that case. In FIG. 14, the horizontal axis represents the voltage applied to the liquid crystal layer 3, while the vertical axis represents the reflectance (efficiency of light use) when the liquid crystal layer 3 is sandwiched between the crossed-Nicol polarizing plates. The reflectance is the gamma characteristic in which the maximum value of the efficiency of light use is normalized to 100% in the red band, green band, and blue band.
As apparent from FIG. 14, the voltages applied to the liquid crystal to provide the highest reflectance in the respective wavelength bands need to have the highest value for the red band, followed by the green band and the blue band, in view of the wavelength dispersion characteristic of the liquid crystal molecules.
Next, FIG. 15 shows the dependence of the reflectance on the voltage in the reflective pixel electrode 2b for each wavelength band when no voltage is applied to the reflective pixel electrodes 2a and a voltage is applied to the reflective pixel electrode 2b in FIG. 13. The horizontal axis represents the voltage applied to the liquid crystal layer 3 through the reflective pixel electrode 2b, while the vertical axis represents the reflectance (efficiency of light use) of the reflective pixel electrode 2b when the liquid crystal layer 3 is sandwiched between the crossed-Nicol polarizing plates. The reflectance is the gamma characteristic in which the maximum value of the efficiency of light use is normalized to 100% in the red band, green band, and blue band shown in FIG. 14.
As apparent from FIG. 15, the gamma characteristic of the reflective pixel electrode 2b shows substantially the same curves in a halftone up to near 4.5 V of applied voltage regardless of the wavelength band.
Since the peak of the efficiency of light use varies in the respective wavelength bands, the efficiency of light use of the reflective pixel electrode 2b shown in FIG. 15 is determined by the comparison with the level of the voltage applied to the liquid crystal layer 3 for providing the highest efficiency of light use in each wavelength band in the all pixel display state shown in FIG. 14.
The comparison between FIGS. 14 and 15 shows that the rate in the light band when the voltage is applied to the liquid crystal to provide the highest efficiency of light use in the all pixel display state (FIG. 14) is different from the rate in the light band in display with the reflective pixel electrode 2b (FIG. 15) in each wavelength band. In the former case, the red band, the green band, and the blue band have the rates of 100%, 100%, and 100%, respectively. In the latter case, the red band, the green band, and the blue band have the rate of 56%, 50%, and 41%, respectively. The all pixel display state (FIG. 14) differs from the display state by the reflective pixel electrode 2b (FIG. 15) in the ratio of the respective wavelength bands in a combined projected image.
FIG. 16 shows the image of a character pattern in which one pixel and one pixel line display white, as a specific projected image. One rectangular in the image represents one pixel. In the image, reference numeral 100 shows pixels in a black display state to which no voltage is applied. Reference numeral 101 shows pixels in a white display state to which a voltage is applied for providing the highest efficiency of light use in the respective wavelength bands.
FIG. 17 shows an image in the all pixel white display state. In the image, reference numeral 101 shows pixels to which a voltage is applied for providing the highest efficiency of light use in the respective wavelength bands.
The comparison between the colors of the projected images in FIGS. 16 and 17 shows that the ratios of light bands in FIG. 17 have a color balance of the red band 1:the green band 1:the blue band 1, and the ratios of light bands in FIG. 16 have a color balance of the red band 5.6:the green band 5.0:the blue band 4.1. Thus, as compared with the color balance in the all pixel white display state in FIG. 17, the color balance in the white character display state shown in FIG. 16 has the particularly low ratio of the blue band, with the result that the character has a color with a shift toward yellow.
FIG. 18 shows the image of a checkered pattern in which pixels in the black display state and pixels in the white display state are alternately arranged. This image has a color balance similar to that of the image in FIG. 16 and has a color with a shift toward yellow as compared with the image in FIG. 17.
As described above, when a character of one pixel and one line or a checkered pattern is output as a projected image, the image has a color different from the color in the all pixel white display state due to the influence of a lateral electric field in the liquid crystal display element. This degrades the quality of the projected image.
In the liquid crystal display element disclosed in Japanese Patent Laid-Open No. 10(1998)-161127, the pretilt angle provided by an alignment film on a first substrate is set to be smaller than the pretilt angle provided by an alignment film on a second substrate to reduce the bad alignment of the liquid crystal molecules due to disclination.
The technique disclosed in Japanese Patent Laid-Open No. 10(1998)-161127, however, relates to the structure of the liquid crystal display element and does not remove the influence of the lateral electric field sufficiently. No disclose has been made on a countermeasure against a change in color balance when a pattern involving the occurrence of a lateral electric field is displayed by a liquid crystal display element in a projector which performs color separation and combination to project an image. Even when the technique disclosed in Japanese Patent Laid-Open No. 10(1998)-161127 is used, the quality of the color of an image resulting from color combination is reduced.